When you create a Studio project by using the New audiobook option, and import a document that incudes chapters, chapters will be detected automatically.
To manage chapters, go to Project options in the top left corner, then select Manage chapters. This will open the Chapters sidebar. You can add a new chapter, using the + button. You can also rename and remove chapters using the Chapter actions (three dots) button, and drag and drop the chapters to rearrange them.
If you started from a blank project, or your project didn't originally include chapters, and you now want to add a new chapter, you can do this by going to Project options in the top left corner, then selecting Manage chapters. This will open the chapters sidebar, and you can add a new chapter using the + button.